Bank notes are received or outputted continually in a cash transaction device, especially in an Automatic Teller Machine (ATM) which has been one of important means for people to obtain cash, and thus recognition and detection on the bank notes is required. Due to an influence of external environment and different properties of the bank notes, the bank notes may be overlapped to each other, that is, overlapped bank notes occur. If it can not be detected accurately whether the bank notes are overlapped, an account dispute may be caused, which results in a serious result.
A most common method for detecting overlapped bank notes is to detect the overlapped bank notes based on the thickness of a bank note. For example, an ultrasonic sensor or a thickness sensor is used to detect the thickness of a bank note. However, information on the thickness of a bank note is very rich due to diversity of information on the surface of the bank note. In this case, detection by the thickness sensor will result in an error or misjudgment on the overlapped bank notes. For example, since the thickness of a security thread area of a bank note is far greater than the average thickness of the other areas of the bank note in most countries, the following problem occurs if the thickness sensor is used to detect whether the bank notes are overlapped. If the non-security thread area passes through the thickness sensor, whether bank notes are overlapped can be detected accurately without misjudgment. If the security thread area passes through the thickness sensor, the thickness sensor detects an exceeding thickness, and thus misjudges the overlapped bank notes, which results in rejection of a large number of bank notes and unstable receiving rate of the bank notes, thus causing a very poor user experience. In this case, although the problem described above can be solved by a method of increasing a thickness reference value or a method of extending a dynamic scope, these methods may bring a new risk that the actual overlapped bank notes cannot be detected, which is very serious.